1. Field of the Invention
The present invention relates to a picture signal processor in which a picture signal (still picture, motion picture) is coded in order to transmit or file it with high efficiency or in which a picture signal spatially sampled is converted into a picture of different resolution.
2. Description of the Related Art
Heretofore, apparatuses for coding picture signals have been used for transmitting or filing the picture signals.
One coding system which is very excellent in terms of coding efficiency and the picture quality is a block coding system which adopts an orthogonal transform.
This system utilizes the fact that, when a signal within a block has been transformed into two-dimensional frequency components, electric power concentrates near a component of low order (for example, a D.C. term corresponding to a mean value). As for the methods of performing the transform, there are the DCT (Discrete Cosine Transform), etc. (refer to, for example, N. Ahmed, et al; "Discrete Cosine Transform", IEEE Trans. Comput., vol. C-23, 1974-1).
A still picture filing apparatus or motion picture transmitting apparatus employing the orthogonal transform has had problems as stated below:
For example, picture resolution conversion including the format change between NTSC and PAL or such an image manipulation process as expansion, compression or rotation has been generally performed in picture-frame unit.
More specifically, the impulse response of an interpolation filter corresponding to the picture conversion is found, and the convolution between the impulse response and an input picture signal is taken. Since, however, this processing method results in a two-dimensional convolution, it has a large amount of processing operations and is realized for the first time by a high-speed processing circuit configured of, for example multiplier units, in large numbers.
Moreover, it is difficult to share a frame memory for storing an original picture and a picture as the processed result thereof, and a frame memory for the interpolation process needs to be separately added. This has led to the disadvantage that the circuit scale for the processing method increases.
An interpolation circuit for the prior-art picture signal processing has often adopted a method wherein a two-dimensional picture signal is multiplied by two-dimensional operators so as to accumulate the products (a method based on the so-called two-dimensional convolution).
This interpolation method is small in the amount of calculations and has been effective as a general interpolation method in the following case:
(1) in a case where the two-dimensional operators are in a comparatively small number, for example, 3.times.3, or
(2) in a case where the relationship between the position of a pixel to be obtained by the interpolation and the positions of the pixels of the original picture is regular (for example, a case where the former pixel lies at the middle point of the latter pixels).
The method, however, has had the disadvantages of an increase in the amount of calculations and enlargement in the circuit scale in the following cases:
(1) in a case where the two-dimensional operators become large in number, for example, where a block average is smoothed and then displayed in the progressive coding of a picture, or
(2) in a case where the pixel to be obtained by the interpolation and the pixels of the original picture are not in a simple positional relationship such as the middle point, as in the conversion between PAL and NTSC signals.